Access to subterranean oil or gas reservoirs may be gained by drilling a wellbore through the earth. In traditional (i.e. substantially vertical) wellbore drilling, advancement of the drill bit is achieved by the application of torsional and axial forces on the drill bit. Torsional forces can be generated at surface by a drilling rig rotating a drill string or downhole by a downhole motor that rotates the drill bit with or without also having rotation of the drill string.
Axial forces may be generated by the incorporation of heavy drill collars in-line with, or as part of, the drill string often in proximity to the drill bit. The axial forces generated by the drill collars can be modulated by changing the surface hook load. When directionally drilling, thrusters can be used to increase the driving force on the drill bit. For the purposes of this disclosure the axial driving force on the drill bit will be referred to as weight-on-bit (WOB).
For a given drilling scenario the operator may determine an optimal range with an upper limit of WOB values, perhaps depending upon the type and manufacture of the drill bit, the depth of drilling and the geological formations to be drilled through. Drilling operators may desire to monitor WOB in order to remain below the upper limit of WOB values to maintain drilling efficiency.
As an additional element of drilling wellbores, drilling fluids are typically pumped from surface through a central bore in the drill string to the drill bit. Drilling fluids may help lubricate and cool the drill bit while drilling in efforts to mitigate deterioration of the drill bit. Drilling fluids may also return to surface, clearing cuttings away from the drill bit.
The monitoring of axial forces on a drill bit during drilling operations, termed weight-on-bit, can assist the operator with maintaining drilling efficiency. For example, if weight-on-bit is greater than a predetermined upper limit the drill bit may deteriorate faster. Replacement of a drill bit may require extraction of the entire drill string, which can be very costly.